I. Field of the Invention
This invention relates generally to biomedical apparatus, and more specifically to a method and apparatus for locating and identifying P-waves in an ECG signal stream, regardless of the existence of PR disassociation.
II. Discussion of the Prior Art
Assessment and analysis of arrhythmias requiring P-wave detection have long been restricted by the lack of effective P-wave detection algorithms. It is recognized that, in surface electrocardiograms, the ratio of R-wave energy to P-wave energy is typically 20-30 dB. That is, the P-wave energy picked up on the surface of the body will typically have 0.01 to 0.001 of the R-wave energy. Furthermore, the P and R-wave spectra have sufficient overlap that traditional linear filtering techniques do not yield sufficient separability. In this regard, reference is made to Pp. 1,113-1,118 of Cardiac Pacing Electrophysiology and Pacemaker Technology containing a chapter entitled "Waveform Analysis of Atrial Electrograms" by A. E. Aubert, et al. Compounding the problem occasioned by the substantial energy difference between P and R-waves and their overlap in the frequency domain is the presence of baseline drift in the ECG signal train and the variety of noise sources which may be encountered during the pickup of the ECG signal stream.
While certain schemes are known for detecting P-waves when such waves are in sinus rhythm with the R-waves, such schemes do not work in the presence of arrhythmias or when the patient's heart is producing disassociated P and R-waves. Here, the P-wave can float into the R-wave and be masked by it.
In a paper by D. E. Gustafson, et al entitled, "A New Technique for Detection of P-Waves" presented at the Midcon Conference on Nov. 6-8, 1979 in Chicago, the authors describe a technique employing the Loeve-Karnunem (L-K) expansion to estimate a noise-free mean QRS complex which is then removed using digital subtraction, from a similar estimate of each QRS complex, leaving a residual signal containing error terms from the L-K approximation, and P-waves. While this process can detect P-waves free of other portions of the cardiac cycle, or proximate to T-waves, it does not work satisfactorily when the P-wave is obscured by the QRS complex. It is conjectured that the L-K reconstruction, fitted term-by-term to the QRS in which a P-wave is contained, allows too many degrees of freedom and thus removes the P-wave energy as well as the R-wave energy during subtraction.
While P-wave detection can be achieved using an esophageal lead or atrial epicardial leads implanted during cardiac surgery for monitoring during critical recovery, these invasive or semi-invasive techniques are generally not acceptable for ambulatory monitoring or for continuous coronary care monitoring where cardiac surgery is not contemplated.
The lack of a reliable non-invasive P-wave detector algorithm has thus hampered development of automatic super-ventricular arrhythmia diagnostic routines, both in ambulatory and coronary care unit monitoring applications. At present, no commercially available computer-based arrhythmia diagnostic systems incorporating atrial activity into the diagnostic logic are available in the marketplace.
Accurate P-wave sensing is required for accuratee arrhythmia and tachycardia assessment and for prompt, safe and effective intervention. The current state of the art uses one or the other of the following two methodologies:
1. Using the R-wave as a fiducial point, a search is made, backward in time, to the expected position of the P-wave and a statistical detection technique is used to make a probabilistic determination of the presence or absence of P-wave at that moment. This technique is not suitable for variable, unexpected or disassociated P-R timing relationships.
2. The use of a separate invasive (endocardial or myocardial) or semi-invasive (esophageal lead) to make a separate determination of the P-wave. Such techniques are acceptable only in selected circumstances where the patient incurs little additional stress or discomfort from their presence, but are not acceptable in general, especially in ambulatory Holter monitoring.